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to withstand pressure conditions even at high temperatures, a property of great value for many uses. It is this rigidity which justifies their use in such installations as the benches carrying gas retorts, furnace crowns, etc.
The degree of fineness of the quartz is quite important in this connection. Coarser grains are undesirable and are apt to give rise to " checking" or cracking due to unequal expansion.
The firing behavior of a siliceous clay as compared with one of the high clay type is illustrated in Figs. 7 and 8, representing respectively a Tennessee ball clay and a siliceous clay from near Ottawa, 111. It will be observed that the former reaches
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0              24              6              8              10            12            14            16            18            20          22 MOLES Si O2 TO  1 MOLE AlsOs-2 Si Og 00        64.90      48.05      38.14      31.62      27.00       23.58      20.90      18.78      17.05       15.60    14.4 % KAOLIN (DEHYDRATED) 0           35.10      51.95       61.86       68.38      73.00      76.42       79.10      81.22       82.95      84.40    85.6 ^QUARTZ																					
FIG. 9.—Seger's softening-temperature diagram.
complete maturity at 1,200°C., undergoing at the same time a contraction in volume of 40 per cent while the latter does not attain minimum porosity even at 1,475°C. with a volume shrinkage of only about 13.5 per cent, beginning with an initial temperature of 1,050°C. Other siliceous clays show these characteristics in a still more marked degree.
It was realized early in the study of fireclays that any addition of free silica to pure clay substance lowered the softening temperature. Thus Seger found ("Collected Writings," p. 434). that a mixture corresponding to the molecular formula A12O3.17-SiO2, composed of kaolin and ground quartz softened at cone 26, while the clay itself fused at cone 35. This composition represents the eutectic between the two components, consisting of 23 per cent of hydrous clay substance and 77 per cent of quartz. Seger's eutectic composition agrees as a whole with the more accurate determinations of Shepherd and Rankin. The curve of Seger is given in Fig. 9.
Siliceous clays therefore are inherently less refractory than the more aluminous pnes and at the same time they are more sensitive to the presence of basic fluxes.